Hubble Space Telescope's ongoing black hole hunt has bagged yet another supermassive black hole in the universe. The compact object equal to the mass of two billion suns lies at the heart of the edge-on galaxy NGC 3115, located 30 million light-years away in the constellation Sextans.
This result promises to open the way to systematic demographic studies of very massive black holes that might once have powered quasars objects that are incredibly small, yet release a gusher of light and other radiation.
Although this is the third black hole confirmation for Hubble, it is the first time the space telescope has demonstrated the feasibility of a powerful black hole identification technique that allows astronomers to directly observe the motions of stars orbiting the black hole.
This technique has previously been used on ground-based telescopes, but its accuracy in detecting and measuring a large central mass is severely limited by the limited resolution of telescopes viewing the galaxy through the Earth's turbulent atmosphere. However, Hubble's detection demonstrates that it is the preeminent telescope for conducting a systematic inventory of normal galaxies to learn how common supermassive black holes are in the universe.
It may turn out that most, if not all galaxies, harbor quiescent black holes at their cores. These black holes may have been active long ago, explaining the abundance of quasars in the early universe.
In 1994, Hubble discovered a 2.4 billion solar mass black hole in the elliptical galaxy M87 by measuring the velocity of a spiral-shaped disk of gas swirling around the galaxy's core. Hubble later found a 1.2 billion solar mass black hole embedded in a gas disk in galaxy NGC 4261. However, because gas disks are rare, other search and detection strategies had to be demonstrated before Hubble could be used to survey other galaxies.
This new technique doesn't rely on the presence of gas disks, but instead looks directly at stars, which occupy the core of all galaxies. Though this technique can be applied to many galaxies, it is also very challenging because it requires careful interpretation of the data because stellar orbits are more complex than a simple rotating disk.
Through careful observations with Hubble's Faint Object Spectrograph, a team of astronomers, led by John Kormendy of the Institute for Astronomy, Honolulu was able to measure the velocities of stars in the galaxy's nucleus which are swirling around the black hole. Follow-up spectroscopic observations were made with the Canada-France-Hawaii Telescope at Mauna Kea, Hawaii. Hubble's high resolution allows astronomers to probe closer in toward the galaxy's nucleus, to obtain definitive data favoring a black hole's presence.
The results show there is far more gravity than would be expected just from stars alone supporting the notion that an extremely massive, dark, and compact object is present. Hubble can't view the black hole directly. Rather, it's presence must be inferred by the effects of its intense gravitational field on the surrounding space. A central black hole will cause stars to orbit the galaxy's core much more rapidly the closer they are to the black hole (just as the orbits of the planets in our solar system can be used to estimate the mass of the Sun).
The team's results will appear in the March 10 issue of the Astrophysical Journal Letters.
Space Telescope Science Institute, Baltimore, MD
University of Michigan, Ann Arbor, MI